A landowner may face a $20,000 bill for a blaze that tore through tinder dry bush and took hours to douse.

The flames sparked accidentally on Monday afternoon in windy conditions on Mill Creek Rd near Whitianga.

Principal rural fire officer for Thames Valley, Del Read, said the fire started in an area where a farm worker was labouring. It spread through scrub, jumped three-metre-wide clay tracks and spread into a stand of gum trees.

Two helicopters with monsoon buckets attended, along with two fire engines and two tankers. They battled the flames from about 4pm and didn't leave the scene until 10pm. Firefighters were still mopping up hot spots yesterday.

Mr Read said the cost of the operation was about $20,000 and they would speak to the landowner about cost recovery.

While Mr Read could not give a clear reason as to why the blaze started, he said farm workers using machinery in dry areas such as the Coromandel Peninsula should be on high alert.

"With farmers working machinery, more often than not at this time of year they can strike the odd stone and send out sparks, from steel striking rock, and they start fires."

The traditional fire ban period on the Coromandel runs from December 20 to February 8.